PEM (Polymer Electrolyte Membrane/Proton Exchange Membrane) fuel cells are electrochemical cells that use hydrogen/hydrocarbon and air/oxygen (collectively “reactants”) to produce water, electricity and heat through an electrochemical reaction.
PEM fuel cells are generally comprised of a MEA (Membrane Electrode Assembly) sandwiched between two separator plates. The MEA is comprised of a proton exchange membrane buffered by two catalyst layers (anode and cathode) and two electrodes (anode and cathode). The proton exchange membrane serves as an electrolyte that conducts hydrogen ions and block electrons; the electrodes serve as electrical terminals that conduct reactants and electrons; and the catalyst layers serve to promote the electrochemical reaction. The separator plates are typically graphite plates that provide structural support, as well as, external access to coolant and reactant inlets/outlets and electrical terminals. Each separator plate contains flow fields or channels etched into the separator plate which serve to deliver reactants to the MEA.
In operation, reactants are supplied through the flow fields of the separator plates to the electrodes. The reactants are then diffused through the electrodes to the catalyst layers. At the anode catalyst layer, hydrogen is split into electrons and protons. The electrons are transported through the anode electrode and the anode separator plate to external circuitry, and return through the cathode electrode to the cathode catalyst layer. The protons are transported through the proton exchange membrane and the cathode catalyst layer, at which point they combine with electrons and oxygen in the cathode electrode to produce water. The water is then transported out of the cathode electrode through the cathode diffusion layer and flow field of the separator plate.
In order to increase output power, a number of PEM fuel cells can be placed in series to form a fuel cell stack. In the fuel cell stack, the separator plates contain flow fields engraved on both sides of the separator plate—one flow field carries hydrogen to the MEA of a first fuel cell while the other flow field carries oxygen to the MEA of a second fuel cell. Since the graphite separator plates are conductive, adjacent fuel cells are electrically connected in series.
The electrochemical reaction produces a substantial amount of heat as a result of losses and inefficiencies in the fuel cell. In order to achieve optimal operating conditions, the fuel cell stack temperature should be maintained within a specified range (typically around 80° C.) that is dictated by design parameters, such as, the MEA properties. Too high temperatures will result in elevated evaporation rates that will dry the MEA, while too low temperatures will slow the electrochemical reaction. To maintain the fuel cell stack at a desired temperature, the excess heat produced from the electrochemical reaction can be removed from the fuel cell stack. To remove the heat produced from the electrochemical reaction coolant may be introduced to the fuel cell stack through a third flow field built into the separator plates. For low power fuel cell stacks (up to a few kilowatts of output power) this coolant can be air. For higher power fuel cell stacks a liquid coolant such as water is typically used. The coolant is circulated in the fuel cell stack to remove heat and is then transported to a heat exchanger where the heat is dissipated from the coolant. The coolant may then be re-circulated back into the fuel cell stack. The use of liquid coolant requires additional components (such as, a heat exchanger, coolant, piping, and control systems) that add to the system size, weight and cost. In addition, the graphite separator plates used in typical PEM fuel cells are heavy and brittle and are not suitable for mass manufacturing purposes.